1. Field of the Invention
The invention relates to rotary drag-type drill bits for drilling in subsurface formations. In particular, the invention is a rotary drag-type drill bit with wearable pads formed of an impregnated diamond/matrix material.
2. Description of Related Art
Diamond impregnated bits may be generally described as being of the kind comprising a bit body having a leading surface comprising a plurality of pads separated by channels for drilling fluid, at least some of the pads each having an abrasive surface including particles of superhard material. The particles of superhard material may be natural or synthetic diamonds, or small bodies of polycrystalline diamond, set in the outer surface of a pad, or the pad may include an outer layer impregnated with superhard particles which, again, may be natural or synthetic diamonds or particles of polycrystalline diamond. It is also possible to combine the two arrangements. Such bits are particularly suitable for drilling through harder subsurface formations.
Hitherto, it has been the usual practice for all parts of the abrasive surfaces of the pads to lie on the same profile. That is to say, at each longitudinal position on the bit, all points on the surfaces of the pads are at the same radius with respect to the central longitudinal axis of the bit. Accordingly, during drilling all of the abrasive surfaces of the bit act on the formation being drilled simultaneously.
In many applications, a lighter set bit (i.e., a bit having fewer superhard particles disposed over the surface of the bit) would be preferable when drilling the upper part of the formation, but a heavier set bit is actually used since such a bit will be required in order to drill through the harder formations likely to be met deeper in the borehole. As a result of having to use the heavy set bit in the softer formations near the surface, the rate of penetration (ROP) of the bit may be lower than that could be achieved by using a lighter set bit.
The present invention therefore sets out to provide a rotary drill bit of the above-mentioned type where the same bit can act as a lighter set bit during initial drilling, but can act as a more heavily set bit as drilling proceeds.
According to the invention there is provided a rotary drag-type drill bit comprising a bit body having a leading surface comprising a plurality of pads separated by channels for drilling fluid, at least some of the pads each having a wearable abrasive surface including particles of superhard material and defining an outer profile of the pad, at least a part of the outer profile of at least one of the pads being disposed inwardly or outwardly of the outer profile of the other pads.
In this specification, for convenience, terms such as xe2x80x9cinwardlyxe2x80x9d and xe2x80x9coutwardlyxe2x80x9d or xe2x80x9cinnerxe2x80x9d and xe2x80x9couterxe2x80x9d refer to positions relative to the center of the bit body; i.e., a point lying on the central longitudinal axis of rotation of the bit.
With this arrangement, during initial drilling through softer formation, most of the removal of formation will be effected by the outermost of the abrasive pads and little or no formation will be removed by the more inward pads. Accordingly, the bit will act as a lighter set bit and good rates of penetration may be achieved.
However, as drilling proceeds and the bit wears, which may occur more rapidly as the bit meets harder formations, the more outwardly disposed pads will wear down more than the inwardly disposed pads so that the inwardly disposed pads will begin to contribute more to the cutting action of the bit, so that the bit effectively becomes heavier set. As drilling progresses to a point where all of the pads wear down to the same level, the bit will then act as a conventional heavy set bit where all the abrasive surfaces lie on the same profile.
The overall profile of a set of inner pads may be generally similar to the overall profile of a set of outer pads, the profiles merely being displaced relatively to one another, for example relatively displaced in the direction of the longitudinal axis of the drill bit.
The abrasive surface of any of the pads may be smoothly and continuously curved so that the outer profile defined by the pad surface follows the contour of the surface itself. Alternatively, the abrasive surface of a pad may comprise higher and lower regions, in which case the outer profile is defined by the higher regions of the pad surface, the lower regions lying inwardly of the outer profile.
Preferably, the drill bit includes a plurality of pads having outer profiles which are disposed inwardly or outwardly of the outer profiles of the other pads, so that under all conditions a plurality of pads engage the formation. For example, some pads may have outer profiles which together define a reference profile, the bit including pads having outer profiles lying inwardly of the reference profile as well as pads having outer profiles lying outwardly of the reference profile.
In known manner, the pads may extend outwardly away from the central longitudinal axis of the bit towards the outer periphery thereof. In this case the outer profile of each pad may be displaced inwardly or outwardly with respect to the profile of the pad on the leading and/or trailing side thereof with respect to the normal direction of rotation of the drill bit. Each pad may extend generally radially outwards from the central longitudinal axis of the bit or may be offset forwardly or rearwardly of a radius of the bit, with respect to the direction of normal rotation of the bit. For example, each pad may extend away from the axis in a spiral.
In any of the above arrangements the outer profiles of the pads, at the outer periphery of the bit, are preferably equidistant from the central longitudinal axis of rotation of the bit so that all of the pads engage the side walls of the borehole.
In any of the above arrangements the particles of superhard material may be set in the outer surface of a pad or the pad may include an outer layer impregnated with superhard particles. Such arrangements may be combined by some pads having particles set in their outer surface while other pads include an outer layer impregnated with superhard particles. Arrangements are also possible where the pad includes both an outer layer impregnated with superhard particles and additional superhard particles set in the outer surface of the impregnated layer.
The superhard particles may be, but are not limited to, natural diamonds, synthetic diamonds, or bodies of polycrystalline diamond material. Where the particles are of polycrystalline diamond they preferably comprise but not limited too thermally stable polycrystalline diamond material.
In known manner, the bit body, or at least the part thereof forming the pads, may be formed from solid infiltrated matrix material.
In known drill bits of the kind first referred to, it is usual for all parts of the surface of each pad to lie on the overall cutting profile of the drill bit so that all parts of the abrasive surface of each pad engage the formation. While such an arrangement may be preferred when drilling some types of formation, some other types of formation may not be efficiently cut by abrasive pads of such configuration. According to another aspect of the invention, therefore, the outer surfaces of the pads are configured in a manner to enhance the cutting effectiveness of the pads with a wider variety of types of formation.
According to this aspect of the invention there is provided a rotary drill bit comprising a bit body having a leading surface comprising a plurality of pads extending outwardly away from the central longitudinal axis of the bit and separated by channels for drilling fluid, at least some of the pads each having an abrasive surface including particles of superhard material, the abrasive surfaces of at least some of the pads being so shaped in circumferential cross-section that some portions of the abrasive surface are disposed inwardly or outwardly of other portions of the surface; i.e. not all portions of the abrasive surface of the pad are at the same radius from the bit axis and do not therefore all lie on the overall cutting profile of the bit.
For example, the surface of a pad may have at least a portion which is inclined to face partly forwardly or rearwardly with respect to the normal direction of rotation of the drill bit, or opposed portions which are inclined to face in both such directions.
Alternatively, the surface of a pad may be formed with alternating ribs and grooves extending along the pads as they extend away from the central longitudinal axis of the drill bit. The ribs and grooves may be generally curved, triangular or rectangular in cross-section, or any combination of these or other shapes.
In an alternative arrangement the surface of a pad may be concavely or convexly curved in circumferential cross-section, the curvature being of smaller radius than the curvature of the overall cutting profile of the drill bit at the location of the cross-section of the pad.
The above described feature of shaping of the cross-section of the pads may be combined with any of the other features of the invention previously referred to.